Laser resonators are used for instance in such laser range finders in which a radiation pulse is transmitted via an optical system to an object, the distance of which is to be determined. Some of the radiation is reflected back again to the range finder where it is detected. The time interval between the radiated pulse and the received pulse is a measure of the distance between said object and the range finder.
In order to keep the divergence of the radiation beam as small as possible it is previously known to increase the length of the resonator cavity by using a number of reflective surfaces between the end reflectors of the cavity allowing the oscillating radiation in the cavity in a so-called "folded" type cavity to be reflected on these surfaces so that a number of successive straight radiation paths are formed. Then the functional length of the resonator cavity is increased without a consequent increase in the exterior dimension of the laser resonator. In order to function satisfactorily it is important that the reflective surfaces of the laser resonator, including the end reflectors, are very carefully positioned. Any deviations have a nonfavourable influence on the output effect as well as on the form of the radiated beam. Laser resonators of the aforesaid type including two parallel ray paths are very susceptible to any mechanical damage due to shocks and vibrations, which are almost inevitable when the laser resonator is used in military applications. This means that repeated adjustments of the optical components are required.
To some extent this disadvantage can be eliminated by using the same reflector both as a front reflector and an end reflector and by using a corner cube prism allowing the radiation to oscillate via this prism, which is characterized by the fact that reflected radiation is parallel to the incident radiation. In previously known laser resonators of the aforesaid type the common end reflector comprises two parts, one totally reflective and one semitransparent for the radiation in question so that some of the radiation is allowed to pass out of the resonator cavity to form the output laser beam.